Near the beginning of the 20th century, it was recognized that a medically useful anatomical image could be obtained when a film containing a radiation-sensitive silver halide emulsion is exposed to X-radiation (X-rays) passing through the patient. Subsequently, it was recognized that X-ray exposure could be decreased considerably by placing a radiographic phosphor panel adjacent to the film.
A radiographic phosphor panel typically contains a layer of an inorganic phosphor that can absorb X-rays and emit light to expose the film. The inorganic phosphor layer is generally a crystalline material that responds to X-rays in an image-wise fashion. Radiographic phosphor panels can be classified, based on the type of phosphors used, as prompt emission panels and image storage panels.
Image storage panels (also commonly referred to as “storage phosphor panels”) typically contain a storage (“stimulable”) phosphor capable of absorbing X-rays and storing its energy until subsequently stimulated to emit light in an image-wise fashion as a function of the stored X-ray pattern. A well-known use for storage phosphor panels is in computed or digital radiography. In these applications, the panel is first image-wise exposed to X-rays, which are absorbed by the inorganic phosphor particles, to create a latent image. While the phosphor particles may fluoresce to some degree, most of the absorbed X-rays are stored therein. At some interval after initial X-ray exposure, the storage phosphor panel is subjected to longer wave length radiation, such as visible or infrared light (e.g., stimulating light), resulting in the emission of the energy stored in the phosphor particles as stimulated luminescence (e.g, stimulated light) that is detected and converted into sequential electrical signals which are processed in order to render a visible image on recording materials, such as light-sensitive films or digital display devices (e.g., television or computer monitors). For example, a storage phosphor panel can be image-wise exposed to X-rays and subsequently stimulated by a laser having a red light or infrared beam, resulting in green or blue light emission that is detected and converted to electrical signals which are processed to render a visible image on a computer monitor. Thereafter, images from storage phosphor panels can be “erased” by exposure to UV radiation, such as from fluorescent lamps.
Thus, storage phosphor panels are typically expected to store as much incident X-rays as possible while emitting stored energy in a negligible amount until after subsequent stimulation; only after being subjected to stimulating light should the stored energy be released. In this way, storage phosphor panels can be repeatedly used to store and transmit radiation images.
By the same token, because storage phosphor panels can be repeatedly used, it is important to protect the phosphor layer from mechanical and environmental damage. Degradation of final images in storage phosphor panels from environmental factors (e.g., humidity, oxygen exposure, liquid exposure, etc.) or for mechanical reasons (e.g., abrasion, jamming, wear and tear, etc.) have been concerns for many years. This is particularly important, for example, in radiographic phosphor panels that are transported in scanning modules and/or handled without protective encasings.
A thick polymeric overcoat layer is typically applied over the phosphor layer to provide adequate protection against mechanical and environmental damage. However, the thickness of the overcoat layer can negatively impact the resolution of the storage phosphor panel. As the thickness of the overcoat layer increases, the amount of stimulating light that is diffused or scattered also increases. Light spreads out as it diffuses, resulting in a loss of spatial resolution and contrast in the resultant image. Thus, the thicker the overcoat layer, the more light diffusion and the lower the resolution. To improve resolution and contrast, thinner overcoats could be employed; however, adequate protection still needs to remain a priority.
Prior solutions to these application problems have been proposed, such as U.S. Pat. No. 6,652,994, which is hereby incorporated by reference in its entirety. This solution involves a complex structure consisting of five layers—a support, a light absorbing layer, a phosphor layer, a reflective layer, and a protective layer. The high number of layers not only increases materials and production costs but also requires a thicker support to sustain the numerous layers above. Moreover, in general, the thinner a phosphor panel at a given amount of X-ray absorption, the better the image quality will be. Consequently, overall thicker panels (e.g., having five or more layers) will usually have poorer image quality as compared to overall thinner panels.
While such structures may have achieved certain degrees of success in their particular applications, there is a need to provide, in a cost-friendly manner, thinner storage phosphor panels having adequately protective overcoat layers with minimal sensitivity to light diffusion.